Electroplating baths containing divalent tin are used widely in industry for plating tin and/or tin alloys onto basis metals. These baths are acidic and are mainly based on acids such as sulfuric, phenolsulphonic, fluoboric, methane sulfonic, or a combination of hydrochloric and hydrofluoric. In all of these baths, a common problem has been the formation of a sludge during operation that results in a loss of divalent tin and excessive clean-up costs. This sludge occurs because, during the plating process, divalent tin has a tendency to become oxidized to tetravalent tin by oxidation at the anode or by oxygen which is introduced into the bath from the surrounding air. Tetravalent tin thus becomes soluble stannic acid which accumulates in the bath to eventually form .beta. stannic acid which is not soluble and which precipitates to form the undesirable sludge. In order to prevent the formation of this sludge, tin must remain in the divalent state.
When plating tin from these solutions onto strip steel using insoluble anodes, the problem is multiplied even further. Oxygen is liberated at these insoluble anodes to further oxidize divalent tin to its tetravalent state. U.S. Pat. No. 4,181,580 describes a process for plating strip steel using insoluble anodes and a method for replenishing tin. Divalent tin is replenished in these plating installations by separately dissolving metallic tin granules in a fluidized bed of acidic plating bath into which oxygen is fed to dissolve the metallic tin. The tin enriched solution is returned to the plating bath thereby replenishing the tin which has been plated out. Excess oxygen in the tin dissolving cell described in this patent can also react with divalent tin to form tetravalent tin; therefore, tin plating machines of this type are particularly subject to formation of tin sludge.
In normal plating installations using soluble anodes and cathode rod agitation, the sludge problem can be minimized. However, when rapid pumping of the solution is used in high speed plating machines, the inclusion of substantial amounts of air into the bath accelerates the oxidation of divalent tin by the oxygen which is present in the air. The sludge problem therefore exists somewhat in normal tin plating installations, is worsened in high speed plating installations, and is further worsened in strip steel machines that use insoluble anodes and tin dissolving cells.
Attempts have been made in the art to minimize sludge formation in these divalent tin baths. A paper by J. McCarthy entitled "Oxidation Characteristics of Tin-Plating Electrolytes," which appeared in the July 1960 issue of Plating magazine, discussed studies of tin oxidation by bubbling oxygen into various tin solutions. U.S. Pat. Nos. 5,094,726 and 5,066,367 disclose methods and solutions for limiting sludge using alkyl sulfonic acid based tin solutions in combination with reducing agents or antioxidants to prevent a buildup of tin.sup.4+. Dihydroxybenzene reducing agents were disclosed to be very effective for this purpose. A recent paper by Chi Pong Ho of the Nanfang Metallurgical Institute appearing in Vol. 24#1 of Materials Protection (January 1991) describes the use of reducing agents based on vanadium pentoxide in divalent tin sulfate-sulfuric acid solutions to limit sludge formation.
Tin plating onto steel strip using acid solutions also results in a continual build-up of iron in the plating bath. The iron content can continue to build until its concentration reaches as high as about 30 g/l. Although the iron interferes only slightly in the tin deposition process, it causes a rapid acceleration of tin sludge formation and a decrease in rate of dissolution of metallic tin in the dissolving cell described above. Any antioxidant used to prevent tin sludge formation in strip plating installations should maintain its usefulness in the presence of this iron buildup in the bath.